I’m assuming (since the OP died) that you mean “why does hair grow to a certain length, then grow again when cut?”
The answer is that hair only grows to a fixed length, then it falls out. If you never cut your hair, your hair will grow to that length, and when the old hairs get too long, they fall out, and are replaced by new hair. You don’t notice an overall difference because at any one time, you have tons of long hairs, and tons of shorter hairs. If you cut your hair often, all the hair is below that maximum length, and so all the hair will grow.